Optically encoded machine readable symbols are well known. Some types of machine readable symbols, such as bar codes, are read by scanning. Typically, an operator brings the scanner into proximity and alignment with the bar code. Omnidirectional bar code scanners, which do not require alignment by the operator, continuously scan the field of view until a successful bar code scan is achieved. Since a bar code has no special finder pattern, a bar code scanner may typically sweep a light beam in a complex geometric pattern until a successful read is accomplished.
However, many other prior art symbologies include a finder pattern, or finder target as it is sometimes called, which is used by the reader to find or locate, i.e. identify the presence or possible orientation of a label containing a machine readable symbol. For example, finder patterns comprising concentric geometric figures including rings, triangles, and hexagons are disclosed in U.S. Pat. Nos. 3,513,320 and 3,603,728. The use of a finder target comprising concentric circles, i.e. a bull's-eye, is also shown in U.S. Pat. Nos. 3,693,154 and 3,801,775. However, these prior art systems employ two separate symbols to identify the machine readable symbol and indicate its position, thereby increasing the complexity of the reader and reducing the data carrying capacity of the label.
More recently, U.S. Pat. Nos. 4,874,936 and 4,896,029 disclose an hexagonal data array using a single finder target comprised of the plurality of concentric circles of contrasting reflectively as part of the label, but separate from the encoded information array. Concentric circles, being a rotational independent target, produce a periodic pattern of known frequency when scanned through the center of the target from any direction. In order to detect the finder target, one-dimensional scan lines are applied to a filter designed to pass the specific frequency of the concentric rings only.
One-dimensional finding of a finder pattern has a variety of serious drawbacks. The first drawback is due to magnification effects. Magnification effects are particularly acute in a belt reader system having fixed focal length optics. As a target is placed at different distances from the image acquisition system (e.g. tall versus short boxes with labels affixed to the top surfaces) the apparent frequency of the target changes. When a label is viewed at different distances from the image acquisition system, the finder target will appear to smaller or larger.
If the size of the finder target changes with distance, then the apparent frequency of the finder target when scanned through the center will also change. The filter used to detect a scan through the finder target center must therefore be designed to accept not only a specific frequency, but a band of frequencies. As the frequency band of the filter is widened, it is more likely that text, graphics, markings, and other optical features viewed by the scanner will excite the finder filter thereby causing false alarms. If there are more false alarms, there is a greater chance that the label will pass the scanning station completely undetected and unread.
The frequency of a scan through the finder target center is also increased by labels that are tilted. Tilting a finder target makes a printed circle appear to be an ellipse, which also changes the apparent frequency of the finder target when scanned at some directions through the center. Furthermore, viewing an elliptically shaped finder target can cause problems in secondary two dimensional tests which look for concentric symmetrical rings.
Another drawback to using a finder pattern of concentric rings is the need to have a sufficient number of rings to excite a filter. The more rings present, the easier it is to find the label and to discriminate against false alarms. However, using more rings reduces the usable area on the label for encoding data. More rings in the same amount of area will result in a label that has very small features. Another drawback of this system is using analog filtering which may require adjustments during manufacture and may be sensitive to environmental conditions, age of equipment, etc.
For example, in the prior art hexagonal data array cited above, the label includes a finder target with six concentric rings occupying a small percentage of the entire label. However, the entire label must have very small features to both encode the desired amount information and have a reliable finder pattern. Small features reduce the depth of field possible. The described system must use powerful illumination, a high resolution imager, a variable focus lens, and a means for sensing the height of the object being scanned. Larger features would allow fixed focus optics, a lower resolution imager, and reduced illumination, but would not provide adequate data density and would require a very large finder target. In general, it is very desirable to increase feature sizes where possible.
Another type of finder pattern used in the prior art is the USD 5 dot code consisting of a row of black squares separated by white spaces. Additionally, the area above and below the row of black squares is also white. The USD 5 finder pattern is space inefficient since it takes up a large percentage of the entire label. Furthermore, because each information bearing cell in the USD 5 label is surrounded by a large amount of non-information bearing white space, the label itself is space inefficient and can only encode a few characters of information.